Channel catfish (Ictalurus punctatus) are widely cultured in commercial fisheries because they are easily managed and have good food quality. Although much research has been performed on raising, stocking, and increasing the nutrition of channel catfish, little research has been directed toward genetic improvements that would increase its value as a cultured species.
One genetic improvement that provides benefits to fish culture is the production of triploid individuals. Triploidy is one type of polyploidy which may be generally described as a specimen with three complete sets of chromosomes versus the usual two sets of chromosomes. A triploid condition has been shown to be beneficial to commercial catfish production because triploid catfish grow larger and exhibit better feed conversion than diploid full-sibs.
Triploid channel catfish have been found to be sterile, however, and natural spontaneous triploidy in fish is rare. Thus, alternative methods for producing triploidy have been developed. For example, triploidy has been induced in certain fish species by cold-shocking fertilized eggs. U.S. Pat. No. 4,489,674 describes a method for 100% triploid conditioning by cold-shocking fertilized eggs at about 5.degree. C. for about one hour approximately five minutes after fertilization.
Tetraploidization and mating tetraploid channel catfish with normal diploids to produce triploid offspring provides an improved method of producing triploidy, as compared to previous cold-shock treatment methods. Such previous triploidization techniques requiring hormonal treatment, sperm and egg handling, and cold shocks are not practical for commercial catfish production because of the time and labor required to produce enough fingerlings for grow-out ponds.
Studies have been made concerning the induction of tetraploid condition by thermal shock during early embryonic development. When heat shocks are applied shortly before first cleavage division, cytokinesis is inhibited. Zygotes undergo two genomic replications with only one cytoplasmic division. C. L. Rieder and A. S. Bajer reported that heat shocks cause depolymerization of tubulin polymers that form the microtubules essential for the formation of the spindle apparatus ("Effect of Elevated Temperature on Spindle Microtubules and Chromosome Movement in Cultured Newt Lung Cells", Cytobios, 18:201-234, 1978). G. Gaillard and A. Jaylet reported in "Mechanisme Cytologique de la Tetraploidie Experimentale Chez la Triton Pleurodeles waltlii Michach," Chromosoma (Berlin), 38:173-184, 1975, that heat shocks are thought to inhibit spindle formation and aster movement.
The use of heat-shocks to induce tetraploidy in Tilapia aurea and rainbow trout, Salmo gairdneri, has been reported in R. J. Valenti, "Induction of Polyploidy in Tilapia aurea (Steindachner) by Means of Temperature Shock" J. Fish. Biol., 7:519-528, 1975; G. H. Thorgaard et al. "Polyploidy Induced by Heat Shock in Rainbow Trout" Trans. Am. Fish Soc., 110:546-550 (1981); and D. Chourrout, "Tetraploidy Induced by Heat Shocks in Rainbow Trout Salmo gairdneri" R. Reprod. Nutr. Develop., 22:569-574, 1982. Resultant embryos, however, were frequently abnormal and at 18 months, no tetraploid rainbow trout were found in a heat-shock group that had produced the tetraploid embryo.
It is, therefore, an object of this invention to provide a method for producing tetraploid channel catfish having little or no anatomical abnormalities in embryos or in growing fry.
It is a further object of this invention to increase the hatchability and the percentage tetraploidy in channel catfish.